Telecommunications networks have traditionally been circuit-switch networks that have transmission paths dedicated to specific users for the duration of a call and that employ continuous, fixed-bandwidth transmission. Due to growth in data traffic created by the Internet and its related technologies, however, telecommunications networks are being moved to a packet-switching transmission model. Packet-switch networks provide a large range of digital services, from data to video to basic voice telephony. Packet-switch networks can allow dynamic bandwidth and may be connectionless with no dedicated path or connection-oriented with virtual circuits and dedicated bandwidth along a predetermined path.
Asynchronous transfer mode (ATM) is a connection-oriented packet-switching technology in which information is organized into small, fixed length cells. ATM carries data asynchronously, automatically assigning data cells to available time slots on demand to provide maximum throughput. Compared with other network technologies, ATM provides large increases in maximum supported bandwidth, designed-in isosynchronous traffic support, support for multiple types of traffic such as data, video, and voice transmissions on shared communication lines, and virtual networking capabilities, which increase bandwidth utilization and ease network administration.
ATM cells are routed through a telecommunications network at high speeds using a switching label included in the cell. The switching label has two sections that define a virtual path (VP) and a virtual channel (VC) in the network through which the cell is routed. The use of virtual paths and virtual channels allows physical bandwidth in the network to be subdivided and separately commercialized.
Because of the low latency and predictability throughput ATM offers, it is capable of providing quality of service (QoS) features. QoS is defined in terms of the attributes of end-to-end ATM connections and is important in an integrated service network, particularly for delay-sensitive applications such as audio and video transmissions, as well as voice-over IP. Other applications in which QoS may be important include traditional data communications, imaging, full-motion video, and multimedia, as well as voice.
Performance criteria for describing QoS for a particular connection include cell loss rate (CLR), cell transfer delay (CTD), and cell delay variation (CDV). ATM traffic is classified as either constant bit rate (CBR) traffic, real-time or non real-time variable bit rate (VBR) traffic, available bit rate (ABR) traffic, and unspecified bit rate (UBR) traffic, depending on the QoS parameters applied to the traffic. CBR and VBR traffic utilize dedicated bandwidth and are intended for real time applications. ABR traffic is intended for non-real time applications which can control, on demand, their transmission rate in a certain range. Like ABR, UBR traffic is intended for non-real time applications which do not have any constraints on the cell delay and cell delay variations.
For CBR, VBR, and other traffic having dedicated bandwidth, transmission slots can be spaced throughout a frame in the transmission line to minimize cell delay variation. However, ABR, UBR, and other types of dynamic bandwidth traffic are evaluated based on transmission line constraints and transmitted in time slots as they become available. As a result, such traffic typically has a high cell delay variation.